The field of the invention is battery powered medical equipment, and particularly, indicators of the remaining battery capacity on such equipment.
Mobile equipment is often battery powered so that it can be moved to any location and used without the need of an electrical power outlet. One such device, for example, is a mobile x-ray unit which is moved rapidly from patient-to-patient in the emergency department of a hospital. Such units must always have enough battery capacity available to produce a diagnostically useful x-ray, and if it is self-propelled, enough capacity to return the unit to its recharging station.
Battery powered medical equipment requires some means for indicating the remaining battery capacity. Typically, this takes the form of a voltmeter which indicates battery voltage. However, as shown in FIG. 1, because the battery voltage drops very little over much of the battery's usable capacity, and drops linearly over an even smaller portion of its usable capacity, this method of indicating battery capacity is not reliable. Normal voltage profile differences between battery manufacturers as well as changes in voltage profiles as the battery is cycled during normal use can result in misleading capacity indications. Such a variation is shown by the dashed line 2 in FIG. 1, where the battery voltage drops slightly at the outset and remains fairly constant until very little charge is remaining.
The difficulty of measuring battery capacity based on voltage is increased where accuracy is required while the battery still has substantial remaining charge. As shown in FIG. 1, when the battery is nearly discharged the voltage drops substantially and a warning indication based on low voltage may accurately indicate the remaining charge shown by cross hatching 3. On the other hand, where the warning indication must be made with a larger remaining charge as indicated by the cross hatching 4, the measurement of battery voltage may be a very inaccurate indication. For example, the measured voltage may properly indicate the remaining charge at dashed line 5 on a new battery, but the same voltage may trigger a much earlier indication at dashed line 6 after the battery is cycled a number of times.